I. Field of the Invention
The present invention relates to a heat sensitive heater wire used for electric heating devices such as surface heating devices.
II. Description of the Prior Art
In the past, a temperature sensor wire, a heater wire, a heat sensitive heater wire or the like used for a surface heating device is constructed as shown in FIG. 1. That is, a conductor 2 for a first electrode is spirally formed on a core thread 1, and a high-molecular weight heat sensitive layer 3, a conductor 4 for a second electrode and an insulating housing 5 are formed in said order. In case of the heater wire, at least one of the electrode conductors is used as a heat generating element wire, and the high-molecular weight heat sensitive layer is used as a temperature fuse. In case of the temperature sensor wire, it is formed into a temperature sensor which detects a change in impedance resulting from the temperature of the high-molecular heat sensitive layer. In this system, the sensor and heater are formed of separate wires, which is called a two-wire system. On the other hand, in case of the heat sensitive heater, one of the inner and outer electrodes serves as a heat generating element wire, and the other serving as a signal wire, which detects a change in impedance resulting from the temperature of the high-molecular weight heat sensitive layer and also has a function as a temperature fuse in response to abnormal rise in temperature. This system is called a single wire system.
These systems have a function for controlling temperature and a function for detecting local overheating, but their heating value per unit length is constant, and their temperature distribution varies with changes in the wiring pattern. The local overheating detecting function is insufficient, and the characteristic thereof greatly depends on the B-constant of the sensor and the wiring pattern. Moreover, the system is large in size and the local detecting function deteriorates as the heater length increases.